There has been known a system in which the frequency range of a speech signal, encoded at a low bit rate and reproduced, is extended on the receiving side without the transmitting side having to send the auxiliary information for band extension (for example, see Non-Patent Publication 1).
Non-Patent Publication 1:
P. Jax, P. Vary, “Wideband extension of telephone speech using hidden Markov model”, Proc. IEEE Speech Coding Workshop, pp. 133-135, 2000.
With this state-of-the-art system, filter coefficients after band extension using HMM (Hidden Markov Model) are retrieved on the receiving side.
On the other hand, the processing for directly extending the band of the narrow-band input signal is unprecedented.
In the state-of-the-art method, shown in the Publication 1, in which modeling by HMM of filter coefficients or the broadband spectral envelope of speech is required, the following problem arises. That is, HMM model parameters need to be determined off-line at the outset from a voluminous speech database in a manner which entails prolonged computing time and increased cost. In addition, retrieval by an HMM model is needed for the receiving side to carry out band extension processing in real time, for which a large volume of calculations are required.
Accordingly, it is an object of the present invention to overcome the aforementioned problem and to provide a method and an apparatus for directly extending the frequency range of a narrow-band input signal. It is another object of the present invention to provide a method and an apparatus for extending the frequency range whereby the band-extended speech of optimum sound quality may be obtained with computational complexity less than that of the state-of-the-art system.